The threat of industrial action is looming at Bus Éireann in a row over rosters.

The National Bus and Rail Union (NBRU) has put the company on notice that if it moves to unilaterally implement non-agreed changes to rosters from Sunday, it will ballot its members for industrial action.

In a separate development, Siptu also informed Bus Éireann in a letter sent on Monday that it too is prepared to ballot its members for industrial action, if it believes the company has broken a Labour Court recommendation agreed earlier this year.

Willie Noone, from Siptu, said the Labour Court recommendation said the rosters and the rotations - effectively what people will be working - would be given six weeks in advance.

"As of last Friday, some locations still didn't have that. That is a clear breach of the Labour Court recommendation," he maintained.

"We said that in circumstances where the company would go ahead and try to implement change on Sunday, that we would immediately have to ballot our members."

He said members around the country were balloted on the recommendations, and members would have to be balloted in the event of any breach of them.

"We have agreed to do this, but we agreed to do it in a certain manner," Mr Noone said.

The company was at the centre of a three-week long strike earlier this year.

The workers had been in dispute with the company over a proposed survival plan.

As part of these recommendations, new rostering arrangements for staff are set to be introduced. However, some issues have now arisen.

The company said: "Bus Éireann has - and continues to be - in dialogue and consultation with unions regarding new rosters.

"Management are currently considering the contents of recent letters received from both Siptu and the NBRU," a spokeswoman said.

"It remains of paramount importance that these rosters and rotations - which improve efficiency - are introduced as planned and previously agreed, given ongoing financial losses."

However, NBRU general secretary Dermot O'Leary insisted that if the company moved to unilaterally implement non-agreed changes to rosters or rotations next Sunday, it would immediately ballot its members.

An implementation date of November 12 was planned, but that was deferred for one week.